Pivotal bone anchor assembly with snap-in-place bushing above a shank head hemisphere

ABSTRACT

A spinal screw assembly providing an adjustable securement of a fixation rod across at least two vertebrae. The assembly includes a pedicle screw having a spherical head portion, a threaded shaft portion and a tool engagement surface in the head portion for use in driving the screw into a vertebrae. The head portion of the screw is positioned in a body member adjacent a curvilinear surface disposed about an aperture in the end of the body member such that the shaft portion of the screw extends therethrough and the curvilinear inner surface abuts and mates with the head portion of the screw so as to define a ball joint therewith. The body member additionally defines a pair of opposed parallel slots therein adapted to receive a portion of the fixation rod and a locking cap bears against the fixation rod to releasably secure the rod within the assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.13/507,857, filed Aug. 2, 2012, now U.S. Pat. No. 8,636,775, which was aContinuation of U.S. patent application Ser. No. 12/154,448, filed May23, 2008, now U.S. Pat. No. 8,298,265, that was a continuation of U.S.patent application Ser. No. 10/848,946, filed May 19, 2004, now U.S.Pat. No. 7,377,923 that claimed the benefit of U.S. ProvisionalApplication No. 60/472,578, filed May 22, 2003 and U.S. ProvisionalApplication No. 60/527,060, filed Dec. 4, 2003, all of which areincorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates generally to an apparatus for internalfixation of the spine and, more specifically to a novel lockingmechanism for a variable angle spinal screw assembly that provides foreasier implantation, a wide range of motion, ease of disassembly foradjustment or replacement of the stabilization rod and eliminatesconventional threaded engagements and the crossover threading, torquingand other problems associated therewith.

Certain spinal conditions, including a fracture of a vertebra and aherniated disc, indicate treatment by spinal immobilization. Severalmethods of spinal joint immobilization are known, including surgicalfusion and the attachment of pins and bone plates to the affectedvertebras. One known device is a bone interface anchor inserted into atleast two spaced-apart vertebras, with a stabilization rodinterconnecting the two or more anchors to stabilize the vertebrasspanned by the anchors. Specifically, a bone screw is received within asocket formed in the anchor. The anchor further includes a channel,extending perpendicular to the longitudinal axis of the bone screw, forreceiving the stabilization rod. The anchor further comprises a threadedportion above the channel. After the bone screw and anchor have beeninserted into the bone material, the rod is placed within the channeland a nut is mated with the external threads of the anchor. The nutapplies a compressive force between the rod and the screw head to firmlyfix the rod between the spanned vertebras and thus stabilize the spinalvertebrae.

During surgical implantation of these prior art stabilization systems,the surgical site is crowded with tissue masses, sponges and othersurgical implements that obstruct access to the anchor threads. Giventhe difficult access, it is possible for the surgeon to cross-thread thenut with the threads of the anchor after the fixation rod is in place.If the threads of the anchor are cross-threaded, the cross-threadedcoupling must be removed and replaced before the surgery can proceed. Inaddition, the threaded fastener (e.g., the nut) is frequently removedand then reinstalled as the surgeon makes progressive bends to contourthe fixation rod. This increases the surgery with each on-off iterationand further increases the chances of cross-threading.

Another problem associated with threaded attachments is the torqueexerted on the anchor during the tightening of the threaded fastenerabout the upper end portion of the fixation device. This torque caninadvertently introduce stress points along the rod, bend the rod oreven loosen the threaded engagement of the anchor in the bone. Theelimination of the conventional threaded attachments in the fixationdevice of the present invention also obviates these problems associatedwith torquing.

The angle at which the anchor screws extend from the vertebra pedicle isdictated by the spinal curvature, the orientation of individual vertebrawithin the spine, and the surgeon's placement of the screw within thepedicle. For example, there is considerable spinal curvature in theregion of the S1-L5 vertebra junction and the angle between thelongitudinal axis of the screws and the vertebra in that region varyover a wide range. Also, it may be necessary to displace one or more ofthe anchors from the spin midline to effectuate maximum spinalstabilization. Thus, the rod-receiving channels are typically notcollinear nor coplanar and, the rod must be shaped or contoured by thesurgeon during the implantation procedure to fit within the channelsalong the spinal column. The prior art systems allow the coupling unitto pivot with respect to the screw over a range of about .+−.20.degree.to .+−.30.degree., providing some margin for the surgeon to place therod within the channel.

One challenge with current variable angle or polyaxial systems isaligning the coupling units in a manner that minimizes pre-insertion rodcontouring while allowing the surgeon maximum range to optimize pediclescrew placement. This is especially challenging when fusing the S1-L5junction. The prior art coupling units allow only a limited range ofmotion with respect to the screw head. The present invention allows afirst range of motion in all directions, but also provides an extendedrange of motion in the medial—lateral—inferior direction (head-to-toe).This extended range of motion, as compared to the prior art, allows thesurgeon additional freedom in locating the screws and eases the assemblyprocess by reducing the requirement for rod contouring.

Thus, the present invention provides an extended range of motion ascompared to the prior art, allowing the surgeon additional freedom inlocating the screws and easing the assembly process by reducing therequirements for rod contouring. The present invention additionallyeliminates the numerous problems heretofore experienced with threadedfasteners. The result is a significantly improved variable angle spinalscrew assembly.

SUMMARY OF THE INVENTION

Briefly, the present invention comprises a variable angle spinal screwassembly for providing an adjustable securement of a stabilization rodbetween at least two spaced-apart vertebrae to effect internal fixationof the spine across the affected vertebrae. The assembly is used with atleast one other such assembly to secure the fixation rod and includes apedicle screw, a body member and a locking cap. The pedicle screw has asubstantially spherical head portion defining a slot therein used todrive the screw into a vertebrae. The body member is generallycylindrical in configuration and is adapted to receive the head portionof the pedicle screw and cooperate therewith so as to define a modifiedball joint to allow variable angular movement of the body member withrespect to the pedicle screw with the threaded shaft portion of thescrew extending through an opening in the inner end of the body member(or lower end as seen from the perspective shown in the drawings). Thebody member additionally defines a pair of opposed parallel slotsaxially disposed in the side wall thereof to receive a portion of thecylindrical fixation rod. The interior walls of the upper portion of thebody member are provided with serrations defining a plurality of axiallyaligned ratchet teeth. The ratchet teeth are adapted to cooperate withopposed mating teeth formed on the outer surface of a locking cap suchthat upon pressing the locking cap downwardly within the body member ofthe assembly against the fixation rod and the rod against the head ofthe pedicle screw, the interlocking teeth will hold the cap in place andsecure the fixation rod in place within the assembly. By rotating thelocking cap with respect to the body member, the ratchet teeth aredisengaged, relieving the pressure of the cap on the fixation rod andthus releasing the securement of the rod.

By providing the body member of the assembly with a rounded interiorsurface about the lower opening therein that mates with the roundedlower surface of the spherical screw head and with concave exteriorsurfaces on the underside of the body member about said opening, theangular orientation of the central axis of the body member relative tothe pedicle screw is widely variable, providing an extended range ofmotion to facilitate surgical installation. Through the cooperation ofthe interlocking ratchet teeth on the body member and locking cap,installation is further facilitated and the disadvantages ofconventional threaded fasteners are obviated.

In preferred embodiments of the present invention, a bushing is employedwithin the body member to better distribute the longitudinal forcesexerted on the pedicle screw. The bushing can be of a generallycylindrical configuration, positioned adjacent the interior side wall ofthe body member and defines a seat for the fixation rod and a bifurcateddepending skirt that abuts and mates with portions of the head of thepedicle screw upon being urged thereagainst by the locking cap pressingdownwardly on the fixation rod. As a result, the force exerted on thescrew is distributed about the head of the screw to improve the lockingsecurement between the screw and the body member.

In addition, by providing a keyed interface between the pedicle screwhead and the body member, the pedicle screw can be inserted into thebone by the surgeon unencumbered by the body member. The body member canthen be aligned with the head of the embedded screw, slid onto and overthe screw head, reoriented so as to mate the inner lower surface of thebody member with the screw head to define the above-described modifiedball joint and the resulting variable angle or polyaxial relationship.Such a keyed interface can be provided by a threaded engagement betweenthe lower end of the body member and fixed screw head by which the bodymember can be screwed onto and over the head of the embedded screw.Alternatively, the screw head and body member opening can be multi-sidedand configured so as to allow the body member to be slid over the screwhead only when the two components are in a given alignment. Once thebody member is slid over the head and rotated so as to misalign therespective sides, the body member is locked onto the screw head and thevariable angle mating relationship therebetween is formed.

It is the principal object of the present invention to provide animproved securement of a fixation rod between two or more spaced-apartvertebrae to effect internal fixation of the spine across the affectedvertebrae.

This and other objects and advantages of the present invention will bereadily apparent from the following detailed description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the variable angle spinal screw assemblyof the present invention.

FIG. 2 is a perspective view of the bone screw portion of the assemblyof the present invention.

FIGS. 3A and 3B are perspective views of the body member of the assemblyof the present invention.

FIG. 4 is another perspective view of the body member of the presentinvention showing the lower surface thereof.

FIG. 5 is a perspective view of the bushing employed in the presentinvention.

FIGS. 6A and 6B are perspective views of a first embodiment of the capof the present invention.

FIG. 7 is a second embodiment of the cap of the present invention.

FIG. 8 is a side view of the variable angle spinal screw assembly of thepresent invention.

FIG. 9A is a sectional view taken along the line A-A of FIG. 8.

FIG. 9B is a sectional view taken along the line B-B of FIG. 8.

FIG. 10 is an exploded view of a modified form of the pedicle screw andbody member employed in the present invention.

FIG. 11 is a perspective view of the modified pedicle screw and bodymember of FIG. 10 shown in the attached position prior to threading thebody member over the screw head to form the mating relationship betweenthe spherical lower portion of the screw head and the interior lowersurface of the body member.

FIG. 12 is an exploded perspective view of another modified form of thepedicle screw and body member employed in the present invention.

FIG. 13 is a representational side view of the embodiment of the pediclescrew and body member shown in FIG. 12 with the body member on the screwin the mating variable angle position.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now in detail to the drawings, the variable angle spinal screwassembly 10 of the present invention comprises a pedicle screw 12, abody member 14, a bushing 16 and a locking cap 18. The assembly 10 isused with at least one other such assembly and a stabilization orfixation rod 19 to connect the assemblies and stabilize the vertebrasinto which the assemblies are inserted. The pedicle screw 12 preferablyemployed in assembly 10 has a spherical head 20 defining a slot 22therein used to drive the screw into the bone. The rounded surface 24defined by the lower portion of screw head 20 rests upon and mates witha rounded interior surface 26 formed in the inner or lower end of thebody member 14 of the assembly 10 so as to form a modified ball jointthat provides the desired variable angular movement of the body memberwith respect to the embedded pedicle screw. The threaded shaft portion28 of screw 12 extends therefrom through the opening 30 in the lower endof body member 14.

The body member 14 of assembly 10 further defines a pair of opposedparallel slots 32 axially disposed in the side wall 34 thereof, whichterminate at their lower ends in curvilinear surfaces 36. The two slots32 are sized to receive the fixation rod therein as shown in thedrawings with the walls 35 defining the slots preferably extendingupwardly beyond the midpoint of the rod and can be inclined slightly toprovide a slight holding force on the rod prior to securing the rod withthe locking cap 18. Thus, during assembly, the surgeon exerts a slightdownward force on the rod, snapping the rod into the transverse channeldefined by the aligned slots 32.

The outer or upper interior surface of side walls 34 of the body member14 both have radially projecting serrations formed therein defining aplurality of axially aligned ratchet teeth 38. The exterior bottomsurface 40 of body member 14 has spaced outwardly extending concavesurface 42 formed therein and a pair of perpendicularly disposed concavesurfaces 44. Surfaces 42 and 44, together with mating surfaces 24 and 26on the screw head and body member of the assembly, provide an extendedrange of motion of the body member 14 with respect to the pedicle screw12. In one embodiment, the range of motion is about .+−.30.degree. inall directions (as measured from the longitudinal axis of the screw) andabout .+−.40.degree. in the inferior-superior direction, the outwardly(as viewed from the screw head) concave surfaces provide the.+−.40.degree. range of motion, for a total motion range of 80.degree.This extended range of motion, as compared to the prior art, allows thesurgeon additional freedom in locating the screws and eases the assemblyprocess by reducing the requirement for a rod contouring.

To secure the fixation rod 19 within the body member 14 of the assembly,locking cap 18 is provided. Cap 18 defines a top portion 48, a pair ofopposed arcuate depending leg portions 50 and a centrally disposeddepending projection 52 equidistantly spaced from leg portions 50.Central projection 52 preferably defines a planar lower or bottomsurface 54. The leg portions 50 of cap 18 each have a plurality ofradially projecting serrations formed therein that define a plurality ofaxially aligned ratchet teeth 56 adopted to engage teeth 38 on theopposed interior side walls 34 of the body member 14, as will bedescribed

A bushing 16 is preferably employed within the body member 14 of theassembly 10 adjacent side walls 34 to better distribute the longitudinalforces exerted on the pedicle screw. Bushing 16 defines a pair ofopposed concave surfaces 60 formed in the upper end of a circular skirt62 so as to define a seat 64 for the fixation rod 19. The lower portionof bushing skirt 62 is slotted at 66 to provide flexibility therein anddefines depending tapered end surfaces 68 adapted to abut opposed sidesof the rounded screw head 20. A pair of outwardly projecting opposedresilient tabs 70 are provided at the upper ends of the bushing 16between concave surfaces 60 that in some embodiments are adapted to bereceived in a snap fitment within a pair of opposed apertures (notshown) formed in the side wall 34 of body member 14 whereupon the rodseat 64 in bushing 16 is aligned with the channel in the body member.Note that in the illustrated embodiment shown in FIG. 3B, for example,the resilient tabs 70 will engage with the body member 14 innercylindrical surface located below the ratchet teeth 38, the illustratedaperture 72 being located in the vicinity of the ratchet teeth 38 thatcooperate with the locking cap 18 and thus at a distance from thebushing 16. Note that only one of apertures 72 is illustrated in FIGS.3A and 3B to better illustrate the configuration of the ratchet teeth38. In an alternative embodiment, the tabs could be removed from thebushing 16 and located on the body member 14 for engagement withapertures or other receiving structure or members formed in opposedsides of the bushing.

To provide a basic stability to the system during initial assembly, thebushing 16 with its slotted lower skirt portion can be configured toprovide a press fitment about the screw head 20 so that the pediclescrew 12, body member 14 and bushing 16 will not move freely prior tothe insertion and securement of the fixation rod. In addition, the upperportion of the bushing could be configured such that the wall surfaces60 defining the rod seat 64 therein extend upwardly past the midpoint ofthe rod and are slightly inwardly inclined. This would provide the sameslight holding force when the rod is pushed into the bushing seat 64that was above described with reference to the channel walls 35 in thebody member 14 of the assembly 10.

Upon securing the bushing 16 in the body member 14 and the fixation rod12 in bushing seat 64, the locking cap 18 is positioned such that thedepending leg portions 50 thereon are aligned with the side walls 34 ofbody member 14. Upon pressing the cap 18 downwardly into body member 14,the ratchet teeth 38 and 56 on the assembly body and cap interlock so asto allow the cap to be pressed downwardly but not retracted. As cap 18is pressed downwardly into the body member of the assembly, the planarbottom surface 54 of the central projection 52 thereon abuts thefixation rod 19 and presses the rod into and against the seat 64 formedon the upper end of bushing 16. The resulting pressure on the bushingcauses the tapered surfaces 68 on the lower end of the bushing to pressagainst the rounded surface of the screw head 20, thereby securing therod in seat 64 and providing a decentralized and evenly distributedforce acting along the longitudinal axis of the screw. Thus, the use ofbushing 16 creates a taper lock between the pedicle screw and bodymember and increases the area of contact therebetween. The result is animproved locking securement over that provided by the earlier describeddirect contact of the fixation rod against the upper end of the screwhead.

The interlocked ratchet teeth will allow the surgeon to tighten theclamping force on the fixation rod by simply pressing downwardly on thelocking cap 18. The teeth will hold the component parts in place. Toadjust or remove the rod 19, the locking cap 18 is simply rotated 90degrees about its longitudinal axis, whereupon the teeth 38 on thedepending leg portions 50 of the cap are aligned with the open slots 32in the body member 14, allowing the cap to be simply pulled upwardlyaway from the fixation rod 19. A hexagonally configured slot 71 isprovided in the top portion 48 of cap 18 to facilitate the rotation ofthe locking cap with a suitably sized mating tool.

In use, at least two of the pedicle screws 12 with the body members 14and attached bushings 16 disposed about the screw are inserted into thevertebra pedicles spanning the vertebra to be fixated. The surgeonpreliminary contours the fixation rod and checks the alignment betweenthe rod and the mating channels formed by the slots in the bushing andbody member of the assemblies. Since additional contouring is usuallyrequired to improve the alignment, the surgeon incrementally adjusts therod shape and checks the fit within the channels until the rod properlyfits in all channels. During the contouring process, a locking cap 18can be mated with one or more of the body member 14 (by pressing the capaxially into the body member to create the interlock between the ratchetteeth on the body member and the cap) to temporarily hold the rod inplace, thereby assisting the surgeon in achieving an accurate fit. Thelocking caps are then easily removable (by rotating the cap a quarter ofa turn to disengage the interlocking teeth), allowing the rod to befurther contoured. Once properly contoured, the rod is inserted into thechannels and a locking cap is pressed tightly into each body member andbushing to secure the rod in place. To effect securement of the rod ateach of the pedicle screw assemblies, it is solely necessary to pressthe locking cap longitudinally into the body member such that the bottomsurface 54 of the central projection 52 on the cap presses against thefixation rod 19, causing the rod to press downwardly against the bushing16, which in turn mates with and presses against the head of the pediclescrew.

A modified form of the variable angle spinal screw assembly isillustrated in FIGS. 10 and 11. This modified form of the assemblyenables the surgeon to insert the pedicle screw in the bone, by itself,unencumbered by the body member. In the prior embodiment, the pediclescrew 12 must be inserted through the body member 14 before the screwcan be driven into the bone. With the body member attached, securementof the screw into the bone can be somewhat difficult. In the modifiedassembly 100, the outer surface of the spherical head portion 120 of thepedicle screw 112 is provided with threads 121, as seen in FIG. 10. Asin the prior embodiment, the upper end of head portion 120 is providedwith a vertical slot 122 used to drive the screw into place. The lowerinterior portion 113 of the body member 114 to be used with the modifiedpedicle screw 112 is provided with threads 115 adapted to engage threads121 on the screw. As a result, the body member 114 can be threaded onto(see FIG. 11) and over the head 120 of the screw 112 after the screw isdriven into place. With the exception of threads 121 and 115, thepedicle screw 112 and body member 114 are identical in configuration tothe screw 12 and body member 14 of the prior embodiment. Thus, after thebody member 114 is threaded onto and over the screw head and is disposedwithin the interior of the lower end of body member 114, as seen in FIG.11, the variable angular relationship therebetween is formed as in theprior embodiment.

A second modified form of the variable angle spinal screw assembly thatenables the surgeon to insert the pedicle screw in the bone, by itself,unencumbered by the body member is illustrated in FIGS. 12 and 13. Asseen therein, the mating threads on the pedicle screw 112 and bodymember 114 have been replaced with mating octagonal surfaces. In thissecond modified assembly 200, the outer surface of the spherical headportion 220 of the screw 212 is provided with an octagonal portion. Theoctagonal portion is comprised of eight contact surfaces 221, one ofwhich (e.g. 221 a) is unequal in length to the remaining surfaces. Thelower interior portion 213 of the body member 214 to be used with themodified pedicle screw 212 is also provided with an octagonallyconfigured portion adapted to engage and mate with the octagonalsurfaces on the screw head. Because of the inclusion of adifferently-sized surface on both the screw and body member, the pediclescrew 212 will only align with the body member 214 in only one position,i.e., where the shortened contact surface on the screw head is alignedwith the correspondingly shortened surface in the lower interior of thebody member. Accordingly, the pedicle screw 212 can again be insertedinto the bone without being attached to the body member 214. After thescrew 212 is driven into place, the body member 214 can be inserted overthe screw head with the octagonal surfaces thereon aligned with thecorresponding surfaces on the screw head. By pressing the screw bodydownwardly, it is completely inserted onto the screw head and the matingoctagonal surfaces are moved out of engagement. Upon rotating the bodymember and pulling upwardly on the body member, such that the head isdisposed within the interior of the lower end of body member 214 thevariable angular relationship therebetween illustrated in FIG. 13 isformed as in the prior embodiments. It is to be understood that thisform of the present invention is not limited to the use of matingoctagonal surfaces. Any polygonal configuration could be employed on thescrew head and body member wherein at least one of the mating surfaceson the screw head and on the body member is correspondingly off-sized orotherwise differently configured from the remaining surfaces on thescrew head and body member.

In another embodiment of the invention, the bushing 16 is not employed.The opposed axial slots 32 in the side wall 34 of the body member of theassembly define a seat for the fixation rod 19. When the locking cap ispressed into the body member with the fixation rod extendingthereacross, the planar bottom surface 54 of the central projection 52again abuts the fixation rod and, in this instance, presses the rodagainst the upper end of the head of the pedicle screw. For suchapplications, the body member and pedicle screw would be sized such thatthe upper surface of the screw would project above the bottom of theseat defined by the axially opposed slots 32 so as to enable the rod topress against the screw and create a rigid, yet adjustable, securementbetween the body member and the pedicle screw. This embodiment can alsobe utilized with the modified forms of the pedicle screw 128 and bodymember 114 shown in FIGS. 10 and 11. In all of these embodiments, thecomponents of the variable angle spinal screw assembly are preferablyformed of titanium.

It should be noted that while the preferred configuration of the lockingcap provides a rounded and flush mounting on the upper ends of the bodymember 14 when the locking cap is fully inserted against the fixationrod, other locking cap configurations could be employed. For example,FIG. 7 illustrates a locking cap having a generally cylindricalperimeter portion in which the ratchet teeth 56 project radiallytherefrom along leg portions 50. This configuration is illustrated inFIG. 1. As a result, the upper end of the locking cap would be inwardlyoffset from the upper end of the body member without adversely effectingthe operation of the variable angle spinal screw assembly. Various otherchanges and modifications also could be made in carrying out the presentinvention.

Although the present invention has been described by way of exemplaryembodiments, it should be understood that many changes and substitutionsmay be made by those skilled in the art without departing from thespirit and the scope of the present invention, which is defined by theappended claims.

What is claimed is:
 1. A pivotal bone anchor assembly for securing afixation rod to a bone of a patient via a closure, the pivotal boneanchor assembly comprising: a bone anchor comprising a shank body havinga longitudinal axis, a partial spherical head at a proximal end with ahemisphere plane perpendicular to the longitudinal axis at a maximumwidth of the partial spherical head, and an anchor portion at a distalend configured for attachment to the bone; a body member having a lowerportion defining a bottom opening, a pair of integral upright armsextending upward from the lower portion to define a first channelconfigured for receiving the elongate rod, and an axial bore centeredaround a centerline axis, the axial bore communicating with a bottom ofthe body member through the bottom opening, and extending upwardlythrough the first channel to tops of the upright arms, the axial borefurther including an interior surface disposed about the bottom openingand configured to receive the partial spherical head of the bone anchorto provide for pivotal movement between the shank body and the bodymember prior to locking the pivotal bone anchor assembly with thefixation rod and closure, the body member including at least one throughaperture extending transverse to the open channel from the axial bore toan outer surface of the body member; and a bushing configured fordownward slidable insertion into the axial bore of the body member, thebushing including: a base portion with a bottom surface configured toengage the partial spherical head of the bone anchor so as to not extendbelow the hemisphere plane when the partial spherical head is positionedin the axial bore with the longitudinal axis of the shank body alignedwith the centerline axis of the body member; a central openingtherethrough configured to provide access to the partial spherical headof the bone anchor; a pair of bushing arms extending upward from thebase portion to define a second channel with a rod support surfaceconfigured to receive and support the fixation rod; and at least oneresilient tab projecting outwardly from the bushing and configured tosnap into the at least one aperture when the bushing is inserted intothe axial bore of the body member, wherein the at least one resilienttab being snapped into the at least one aperture is configured toinhibit the bushing from slidably moving back upward and out of theaxial bore and to inhibit rotation of the bushing with respect to thebody member, so as to maintain the second channel of the bushing inalignment with the first channel of the body member.
 2. The pivotal boneanchor assembly of claim 1, wherein the partial spherical head of thebone anchor is positioned within the axial bore of the body member priorto the bushing being inserted into axial bore.
 3. The pivotal boneanchor assembly of claim 1, wherein the bushing is configured to bedownwardly displaceable within the axial bore after the at least oneresilient tab is snapped into the at least one aperture.
 4. The pivotalbone anchor assembly of claim 1, wherein the bottom surface of thebushing is configured to flexibly abut the partial spherical head of thebone anchor.
 5. The pivotal bone anchor assembly of claim 4, wherein thebase portion of the bushing is slotted.
 6. The pivotal bone anchorassembly of claim 1, wherein the at least one through aperture furthercomprises a pair of opposed through apertures formed through a sidewallof the body member, and wherein the at least one resilient tab of thebushing further comprises a pair of outwardly projecting opposedresilient tabs configured to snap into the pair of opposed throughapertures when the bushing is inserted into the axial bore of the bodymember.
 7. The pivotal bone anchor assembly of claim 6, wherein the pairof outwardly projecting opposed resilient tabs project outwardly fromthe upper end of the bushing.
 8. The pivotal bone anchor assembly ofclaim 1, wherein the bushing arms are configured to extend upwardly pasta midpoint of the fixation rod when the fixation rod is positioned intothe first channel of the body member and the second channel of thebushing.
 9. The pivotal bone anchor assembly of claim 8, wherein thebushing arms are inwardly inclined so as to hold the fixation rod in thesecond channel of the bushing upon a downward pushing of the fixationrod into the second channel.
 10. The pivotal bone anchor assembly ofclaim 1, wherein the at least one resilient tab includes an uppersurface perpendicular to the longitudinal axis and configured to preventremoval of the bushing tab from within the aperture by translation awayfrom the base portion of the body member.
 11. The pivotal bone anchorassembly of claim 1, wherein the interior surface of the axial boredisposed about the bottom opening further comprises a rounded interiorsurface.
 12. The pivotal bone anchor assembly of claim 1, wherein thebottom opening of the body member includes at least a portion of athread.